In a wide variety of machining operations, such as those performed by milling, routing or drilling machines, it is often desirable and necessary to rigidly support a generally flat workpiece in an elevated fashion above the machine tool bed or work table to allow the tool to penetrate the workpiece without interferring with the machine bed. Examples are numerous and include those where a workpiece has to be milled along its edges by a milling cutter which moves totally around its periphery, or where holes need to be drilled completely through a workpiece or where large openings have to be formed by plunging a routing cutter completely through the workpiece and then moving it through a selected closed path. If the workpiece were positioned directly on the machine bed when performing operations such as these, the tool would inevitably engage the bed, damaging the tool, the bed or both. With the workpiece elevated, a safe degree of clearance is provided between the cutting device and the machine tool bed even when the cutting device projects completely through the workpiece.
In the past, the necessary clearance has been provided by the use of a "spoil-plate" interposed between the machine tool bed and the workpiece. Such an arrangement is diagrammatically illustrated in FIG. 1 where a workpiece 10 is on a spoil-plate 12, which in turn is positioned on the machine bed or work table 14. The work table 14 itself is constructed with vacuum holes 16 which are connected on one end to a vacuum source through an on/off valve 18. The vacuum holes 16 communicate with the top surface of the work table 14 so that the applied vacuum pulls the spoil-plate into tight engagement with the work table. The spoil-plate 12 is specially designed with an undercut recess 20 and with vertical holes 22 communicating upwardly to a top recess 23. In effect, the holes 22 transfer any applied vacuum to the undersurface of the workpiece so that it is held tightly against the spoil-plate, which in turn is held by the vacuum to the work table. The cutter 24 when projecting through the workpiece will drill into or route through the spoil-plate material but will not, of course, touch or damage the table 14. FIG. 1 is somewhat oversimplified; in practice recess 20 and its through holes 22 must be designed to provide adequate and adequately distributed vacuum to hold the workpiece, while at the same time being coordinated to the machining program to avoid interferring with the vacuum hold down.
This type of prior art technique has a number of disadvantages. For instance, the through holes 22, the channel 20 and the upper recess have to be carefully and accurately located so that they are not in regions through which a cutting tool 24 will pass when the workpiece is machined with the cutter projecting completely through the workpiece; otherwise there is the danger that the cutting tool 24 will machine through the spoil-plate's vacuum system, effectively destroying it and causing undesirable workpiece displacement. Hence, the spoil-plate has to be made as a separate fixture especially designed and tailored to the particular workpiece and the machining program for it.
There hence exists a need for a more efficient and convenient arrangement for the vacuum chucking of workpieces, which is not subject to the above problems faced by the prior art.